I saw this in the EU forums as well but wasn’t able to locate it on the US side, over in the Customer Support forum, so I recently posted about it there. Just found your thread here when I came to submit it as a bug report.
This is still a problem, and remix is almost half over. There was a blue post quoted in a wowhead article (the blue post appears to have been deleted, but it’s still in the wowhead article) stating that remix characters would be able to join guilds and complete guild achievements during the event.
It appears not only is it not working, but also not working for retail characters either. And given the announcement that pre-patch is going to launch on July 30, I think it’s fair to say the GM quoted above was misinformed. There is no reason for the Classy achievements to not be working for level 70 characters, whether they are Remix or retail. Leveling alts was one of the major selling points of the event; I imagine a lot of people were aiming for the Classy achievements. Can this bug be expedited so that people get the credit they have earned before TWW pre-patch/launch officially moves the goalposts from 70 to 80?